1. Field of the Invention
The present invention relates to radio frequency communication receivers, systems and methods employing ultra wide band (UWB) signaling techniques. More particularly, the present invention relates to systems, methods and computer program product configured to remove in a UWB receiver xe2x80x9cnarrowbandxe2x80x9d interference from a UWB signal.
2. Description of the Background
Wireless communication systems operate on the principle of using a transmitter that is configured to take data and send the data to an amplifier and antenna, which converts the data from electrical signals into electromagnetic radiation. This electromagnetic radiation propagates through the air, or other medium, and is converted from electromagnetic radiation back into an electric current (or voltage) by a receive antenna coupled to a receiver. The electrical signals coupled into the receive antenna are typically very small and therefore are usually amplified before being sent to a detector for converting the electrical signals into digital information (or the type of format employed by the source signal).
Accordingly, the receiver converts the energy that is passed from the antenna into an electrical form and then isolates the useful information contained within the energy coupled from the antenna to produce a useful output representative of the input signal. A problem arises if an unintended signal, particularly a strong signal, is coupled into the antenna at the same time as the desired signal. In this case, the unintended signal, if it overlaps in frequency with the intended signal, will tend to xe2x80x9cjamxe2x80x9d the reception of the intended signal, thereby reducing reception quality. Furthermore, even if the unintended signal is not coincident in frequency with the desired signal, the unintended signal may nonetheless drive a low noise amplifier (LNA) into a saturation mode, thereby creating unwanted high spurious signals. When this LNA is saturated, the amount of gain imparted by the LNA is reduced and the LNA creates intermodulation products and harmonics, which degrade the reception of the intended signal.
In narrowband communication systems one technique for avoiding the saturation of a front end amplifier by radio frequency interference (RFI) is to design the radio front end with a transfer function that is matched to the desired signal and has a bandpass characteristic centered around the intended signal, but excluding the unintended radio frequency interference (RFI). However, such techniques are not suitable if the intended signal is spectrally broader (but still not UWB), such as in a conventional spread spectrum system, much like a CDMA system or other direct sequence spread spectrum system, or even a frequency hopping system. It is worth mentioning that conventional xe2x80x9cwidebandxe2x80x9d spread spectrum signals are still relatively xe2x80x9cnarrowbandxe2x80x9d with respect to UWB signals.
Another way that a receiver front end can deal with relatively large xe2x80x9cin bandxe2x80x9d interferers is to employ automatic gain control (AGC), so that the amount of gain in the amplifier coupled to the antenna is reduced if exposed to relatively large signal levels from either intended or unintended RFI. However, the problem arises that the unintended RFI may be sufficiently high such that the desired signal is suppressed to a level beneath the dynamic range of the low noise amplifier (LNA) or of the subsequent processing circuitry. In this case it is said that the instantaneous dynamic range of the radio front end is less than the ratio of RFI to intended signal strength.
Another technique for dealing with in-band RFI in broadband communication systems is to first detect and then suppress unintended RFI. However, such systems usually require detection of the interfering signal to distinguish an interferer from an intended signal and special cancellation circuitry dedicated to the function of xe2x80x9cnotchingxe2x80x9d or uniquely suppressing the unintended RFI. Inserting notch filters into the passband creates not only detrimental insertion loss, thus increasing the noise for the radio front end, but also introduces phase distortion into the received signal thus limiting the effectiveness of such systems. Furthermore, such systems are usually not adaptive because it is difficult and expensive to adjust the center band of notch filters based on the particular interfering signal at any given time.
Spread spectrum communication systems have a predetermined amount of xe2x80x9cprocessing gainxe2x80x9d which relates to the amount of redundancy in a transmitted signal. In direct sequence spread spectrum communication systems, this amount of redundancy materializes in the form of a much broader bandwidth used to communicate the signal than is necessary if simply the information itself were transmitted (in a xe2x80x9cnarrowbandxe2x80x9d modulation format). Accordingly, the receiver, when applying the spreading code to the received signal so as to xe2x80x9cdespreadxe2x80x9d the signal also applies the spreading code to the interference, but because the interference does not coherently combine with the spreading code, the interference is reduced in power and the spread signal is despread by the amount equal to the processing gain. More detailed descriptions of spreading techniques and systems for employing spread spectrum communications is described in xe2x80x9cSpread Spectrum Design LPE and AJ Systemsxe2x80x9d, by David L. Nicholson, Computer Science Press, 1987, ISBN0-88175-102-2, the entire contents of which being incorporated herein by reference.
FIG. 1 is a block diagram of a conventional receiver. The receive antenna 100 converts an incoming wireless radio frequency signal into an electrical signal. The bandwidth and passband of the radio front end circuit 102 is matched to the incoming signal so as to extract the desired signal from out-of-band noise. The receiver also includes mixer 104 and local oscillator 106. The output of mixer 104 is an intermediate frequency (IF) signal. The intermediate frequency detector 108 amplifies and band-pass filters the IF signal and outputs it to the RFI suppressor 110 which helps to notch RFI from the signal. In spread spectrum signals, the RFI is easily isolated and extracted because its energy is concentrated out-of band or in a small spectrum range. Next are a second mixer 112 and local oscillator 114, which provide a baseband signal to a baseband processor 116.
Conventional UWB communication systems transmit energy over a much larger bandwidth than normal xe2x80x9cnarrowbandxe2x80x9d or even spread-spectrum communication systems. Accordingly, it would be expected that the number of narrowband signals to be encountered by such UWB systems would be relatively high. Examples of such UWB systems include deRosa (U.S. Pat. No. 2,671,896), Robbins (U.S. Pat. No. 3,662,316), Morey (U.S. Pat. No. 3,806,795), Ross and Mara (U.S. Pat. No. 5,337,054) and Fullerton and Kowie (U.S. Pat. No. 5,6777,927).
In a conventional receiver, RFI suppression is done prior to baseband processing, as shown in FIG. 1. However, as recognized by the present inventors this is particularly difficult when there are multiple RFI signals present, and the power spectral density of the UWB signal is so low with respect to the RFI as is the case with UWB systems.
The best papers known to the inventors on UWB RFI extraction have been for radar applications. Examples are: T. R. Miller, J. W. McCorkle, and L. C. Potter, xe2x80x9cRFI Suppression for Ultra-Wideband Radarxe2x80x9d, IEEEE Transaction on Aerospace and Electronics Systems, vol. 33, no. 4, October 1997, herein incorporated by reference, and the group of papers in Algorithms for Synthetic Aperture Radar Imagery II, D. A. Giglio (ed.), SPIE Vol. 2487, Orlando Fla., April 1995, herein incorporated by reference. These approaches could not be used for an inexpensive and low power communication system because the sample rates used were high enough to capture the spectrum of the radar without aliasing, and the processing was done on bursts of data (i.e. in a radar mode where a burst covered a certain segment of range) not continuously as is the case with a communications system. Indeed, banks of the highest speed digital signal processing available had to be used in order to process the radar signals.
The challenge, as presently recognized, is to correctly receive an intended, transmitted UWB signal at the UWB receiver in the presence of narrowband interference signals in a post-correlation portion of the receiver circuitry where the processing load is small enough to be done in an inexpensive and low power communication system.
Consistent with the title of this section, only a brief description of selected features of the present invention is presented. A more complete description of the present invention is the subject of this entire document.
An object of the present invention is to provide a UWB receiver that includes an RFI extraction mechanism that suppresses at baseband, unwanted in-band RFI without adversely affecting the reception of the intended UWB signal.
Another object of the present invention is to provide a UWB receiver that includes an RFI extraction mechanism that estimates, in the baseband signal after correlation, the intended UWB signal.
Another object of the present invention is to provide a UWB receiver that includes an adaptable RFI extraction mechanism, as well as a method and computer program product, for extracting the RFI in the baseband signal after correlation.
Another object of the present invention is to provide a UWB receiver that passes UWB signals, but suppresses carrier-based modulated signals (i.e. narrowband signals).
Another feature of the present invention is to address the above-identified and other deficiencies of conventional communication systems and methods.
These and other objects are accomplished by way of a UWB receiver that employs a matched filter correlator, an analog to digital (A/D) converter that samples the UWB signal after correlation and an RFI extraction mechanism to identify the RFI signal after the correlation process and remove the RFI.
While several embodiments are disclosed herein, one embodiment would include an RFI removal filter after the A/D converter that passes the desired data signal, but cancels an RFI signal. Another embodiment would include an RFI removal filter and a bit detector after the A/D converter to cancel the RFI signal and extract the transmitted information. Another embodiment would include the same RFI removal filter and bit detector along with an RFI estimator for adapting the filter to cancel observed RFI.